Definition of Quater in die (on prescription)

Quater in die (on prescription): Seen on a prescription, qid (or
q.i.d.) means 4 times a
day (from the Latin quater in die). The abbreviation qid or q.i.d. is also
sometimes written without a period in capital letters as "QID".
However it is written, it is one of a number of hallowed
abbreviations of Latin terms that have been traditionally used in
prescriptions to specify the frequency with which medicines should be
taken.

Other examples include:

q.d. (qd or QD) is once a day; q.d. stands for "quaque die"
(which means, in Latin, once a day).

b.i.d. (or bid or BID) is two times a day; b.i.d. stands for "bis
in die" (in Latin, 2 times a day).

t.i.d. (or tid or TID) is three times a day ; t.i.d. stands for
"ter in die" (in Latin, 3 times a day).

q_h: If a medicine is to be taken every so-many hours, it is
written "q_h"; the "q" standing for "quaque" and the "h" indicating
the number of hours. So, for example, "2 caps q4h" means "Take 2
capsules every 4 hours."